1. Field of the Invention
The invention relates to an installation for producing imagery by nuclear magnetic resonance (NMR) and relates more particularly to wave-shaping of the excitation radiofrequency signal in order to obtain a selected cross-section having the desired characteristics, especially a so-called cross-sectional plane (namely a "flat slice" of predetermined thickness of the body to be examined) having limits in space which are as accurate and sharply defined as possible.
2. Description of the Prior Art
A conventional nuclear magnetic resonance (NMR) imaging installation comprises a magnet for producing a uniform magnetic field in a spatial region in which the body to be examined is placed and means for superimposing on said uniform field a field gradient which is oriented in a chosen direction in space, the characteristics of the gradient being modified during successive sequences of data acquisition. Radiofrequency emission means are provided for producing in said spatial region a rotating field which is capable of initiating spin flip in the case of certain nuclei (mostly hydrogen atoms) of the body to be examined. The conditions of nuclear magnetic resonance in which spin flip can be observed are determined by the local value of the magnetic field and the frequency of the excitation electromagnetic wave. In the case of hydrogen, for example, all the conditions of resonance are satisfied for a radiofrequency emission at 6.4 MHz and a magnetic field of 1500 Gauss or for a radiofrequency emission of 64 MHz and a magnetic field of 15000 Gauss.
The superposition of the constant magnetic field and of a gradient in the aforementioned spatial region limits the conditions of resonance to a thin cross-sectional plane or in other words to a selected cross-section of the body to be imaged, with the result that the spins of the nuclei of this cross-section which are being studied are the only ones to flip and consequently the only ones subsequently to re-emit a measurable radiofrequency signal. By carrying out a large number of emission-reception sequences, the set of data thus collected is sufficiently large to reconstruct the image of the cross-section. In order to ensure that the image is of good quality, a useful attempt can be made to obtain a cross-section having boundaries which are as sharp as possible. This entails the need to utilize emission-reception sequences such that the atoms located in the vicinity of the body-section plane whose reconstructed image is to be obtained are not excited or at most only weakly excited in order to ensure that they make the smallest possible contribution to the re-emitted radiofrequency signal. In actual fact, the "shape" of the selected cross-section varies as a function of the envelope of the radiofrequency signal which produces flip-over of spins.
It has always been assumed up to the present time that the "shape" of the selected cross-section (which can be defined as the density of excited spins along an axis perpendicular to the cross-sectional plane) was the Fourier transform of the "shape" of the radiofrequency excitation pulse or in other words the envelope waveform of this signal. This assumption has dictated the choice of an envelope of cardinal-sine shape, where the cardinal-sine of ##EQU1## represents the inverse Fourier transform of a rectangular wave. A rectangular wave having vertical sides (leading and trailing edges) is in fact the image of an optimum transition between the selected cross-sectional plane and the rest of the spatial region in which the magnetic field prevails. However, the results have in fact proved disappointing and the shape of the selected cross-section obtained with a radiofrequency pulse of the cardinal-sine type is not fully satisfactory.